Catalysis is literally the lifeblood for many industrial/commercial processes in the world today. The most important aspect of a catalyst is that it can increase the productivity, efficiency and profitability of the overall process by enhancing the rate, activity and/or selectivity of a given reaction. Many industrial/commercial processes involve reactions that are simply too slow and/or inefficient to be economical without a catalyst present. For example, the process of converting natural gas or methane to liquid hydrocarbons (an extremely desirable process) necessarily involves several catalytic reactions.
The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is catalytically converted to carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas intermediate is catalytically converted to higher hydrocarbon products by processes such as the Fischer-Tropsch Synthesis or to other chemicals by processes such as an alcohol synthesis. For example, fuels such as hydrocarbon waxes and liquid hydrocarbons comprised in the middle distillate range, i.e., kerosene and diesel fuel, may be produced from the synthesis gas.
Current industrial use of methane as a chemical feedstock for syngas production proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming or dry reforming. Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, the reaction proceeding according to Equation 1.CH4+H2OCO+3H2  (1)
The catalytic partial oxidation (“CPOX”) of hydrocarbons, e.g., methane or natural gas, to syngas has also been described in the literature. In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a more preferable H2:CO ratio of 2:1, as shown in Equation 2:CH4+1/2O2CO+2H2  (2)
The H2:CO ratio for this reaction is more useful for the downstream conversion of syngas to fuels or to chemicals such as methanol than is the H2:CO ratio from steam reforming. However, both reactions continue to be the focus of research in the world today.
As stated above, these reactions are catalytic reactions and the literature is replete with varying catalyst compositions. The catalyst compositions typically are comprised of at least one catalytically active metal, such as a Group VIII metal. Many catalyst compositions also have other promoters present. Catalytic metals are typically selected based on their activity and selectivity towards a particular reaction. Further, the catalyst compositions typically include particular support materials such as alumina, silica, titania, etc., that can also enhance the catalyst activity.
After a period of time in operation, a catalyst will become deactivated, losing its effectiveness for catalyzing the desired reaction to a degree that makes the process uneconomical at best and inoperative at worst. This process is generally known as “aging.” The more aged a particular catalyst the less efficient the catalyst is at enhancing the reaction, i.e., less activity it has. At this point, the catalyst can be either replaced or regenerated. However, replacing a catalyst typically means discarding the deactivated catalyst. Even if a fresh replacement catalyst is ready and available, a single syngas reactor will typically have to be shut down and offline for days to weeks. The time delay is due at least in part to the time required for simple cooling and heating of the reactor.
In addition, a discarded catalyst represents a loss of expensive metals. Alternatively, the user may send the catalyst back to the supplier for recovery of expensive metals, such as Rh, Pt, Pd, etc. However, the recovery process involves dissolving the multi-component catalyst and subsequent separation of the active components from the mixed solution. The chemistry is complex and costly, more importantly, it involves bulk amounts of harsh chemicals that ultimately must be discarded and the use of landfills for such disposal is problematic. For example, the environmental protection agency (EPA) “Land Ban” imposes restrictions on disposal because these harsh chemicals can release toxins into the environment. For all of these reasons, regeneration is preferred over replacement.
However, regeneration has problems as well. Like replacement, regeneration typically requires some downtime resulting in a decrease in production. In addition, regeneration may not be available for every deactivated catalyst. Catalyst systems can become deactivated by any number of mechanisms. Some of the more common deactivating mechanisms include coking, sintering, poisoning, oxidation, and reduction. The process chiefly responsible for deactivation varies among catalyst systems. Some catalysts that have been deactivated can be regenerated and/or the deactivation reaction can be reversed. However, many regeneration processes are not economically feasible.
Sintering as a cause of deactivation traditionally has been viewed as a non-reversible phenomenon, since a sintered catalyst is particularly difficult to regenerate. In terms of synthesis gas catalysts, sintering is usually the result of the high temperatures within the catalyst bed. The syngas reactions achieve very high temperatures during operation. Temperatures within a syngas catalyst bed typically reach temperatures in excess of 1000° C. Sintering for syngas catalysts is therefore practically unavoidable. There are similarly potential deactivation issues with other catalytic partial oxidation reactions that take place at high temperature.
Because regeneration has traditionally been so difficult, the active metals are typically dissolved and recaptured for use in new catalyst batches. However, research is continuing on the development of more efficient syngas catalyst systems and catalyst systems that can be more effectively regenerated. To date there are no known methods that are economically feasible for regenerating a partial oxidation catalyst, such as a syngas catalyst.
Hence, there is still a great need to identify new regeneration methods, particularly methods that are quick and effective for regenerating deactivated partial oxidation catalysts without having to dissolve the catalyst components and without significant downtime or loss of production.